Sphingosine (SPN) is a long chain unsaturated amino alcohol of the formula C.sub.18 H.sub.37 O.sub.2 N found in cell membranes and in high concentration in nervous tissue. Sphingosine and sphingoid base (a long chain aliphatic base comprising a 1,3-dehydroxy-2-amino group at a terminus and derivatives thereof) have been implicated as inhibitors of protein kinase-C (PK-C) and EGF receptor-associated tyrosine kinase (EGF-RK) (Hannun & Bell, Science, 235, 670, 1987; Hannun, JBC, 261, 12604, 1986; Kreutter et al., JBC, 262, 1632, 1987).
Protein kinase-C activity is related closely to cell growth and recent studies indicate that increased tumorigenicity is correlated with over expression of PK-C.sub..beta.1 and PK-C.sub..gamma. in some experimental tumors (Housey et al., Cell, 52, 343, 1988; Persons et al., Cell, 52, 447, 1988). A mutant PK-C.sub..alpha. induces highly malignant tumor cells with increased metastatic potential (Megidish & Mazurek, Nature, 324, 807, 1989). It would appear that aberrant expression of PK-C may relate to tumor progression.
Recent studies indicate that phospholipids, sphingolipids and metabolic products thereof have an important role in the modulation of transmembrane signaling through PK-C and other membrane-associated kinases, such as EGF receptor-associated tyrosine kinase (Hakomori, JBC, 265, 18713, 1990). For example, PK-C activity is promoted by diacyl glycerol and inhibited by sphingosine (Hannun & Bell, supra; Hannun & Bell, Science, 243, 500, 1989; Merrill & Stevens, Biochim Biophys Acta, 1010, 131, 1989).
Sphingosine did not inhibit PK-C in vitro or at concentrations below 100 .mu.M and did not exhibit a stereospecific effect on PK-C (Igarashi et al., Biochem, 28, 6796, 1989). Many of the studies described above employed sphingosine obtained from a commercial source (for example Sigma Chemical Company) and the preparations contained various impurities including 3-O-methylsphingosine, 5-O-methylsphingosine and N-methylsphingosine. The impurities are likely to result from the process of preparation, namely methanolysis of sphingomyelin or cerebroside. Furthermore, in the sphingosine backbone, the D-erythro configuration about the chiral carbons is often converted to the D-threo configuration.
Igarashi et al. (supra) found that the inhibitory effect of sphingosine on PK-C activity is due to: (1) the stereospecific configuration of C1 to C3 (D-erythro configuration required); (2) presence of a long-chain aliphatic group; and (3) perhaps most essential, a negative charge at the primary amino group at C2. If the amino group was N-acetylated, the PK-C inhibitory activity was abolished since the negative charge of an amino group was eliminated by acetylation. However if the anionic character of the amino group was enhanced by N-methylation, the stereospecific PK-C inhibitory activity was enhanced.